Adding jemalloc to Rails apps on Heroku

How to enable jemalloc on Heroku for Rails apps

jemalloc is a malloc implementation developed by Jason Evans which is known to improve memory consumption of Rails apps, especially on Heroku. By default Ruby uses malloc from C to manage memory but it can run into memory fragmentation issues.

Whereas jemalloc describes itself as a malloc implementation which tries to avoid memory fragmentation. A lot of people have successfully tested jemalloc in production apps deployed on Heroku to verify that it reduces memory usage compared to the default memory management that comes with Ruby.

Enabling it in a Rails app on Heroku consists of following steps.

Add jemalloc buildpack

heroku buildpacks:add --index 1

Enable jemalloc

This can be done in two ways. Either we can set the environment variable, JEMALLOC_ENABLED to true.

heroku config:set JEMALLOC_ENABLED=true

Alternatively, we can add prefix to the processes listed in the Procfile.

# Procfile

web: bin/puma -C config/puma.rb
worker: bundle exec sidekiq -q default -q mailers -c ${SIDEKIQ_CONCURRENCY:-5}
Note that setting the JEMALLOC_ENABLED environment variable will enable jemalloc for all processes of your app, where as adding prefix in Procfile can give you control over for which processes you want to enable it.

After this is done, deploying the app on Heroku will enable jemalloc and we can monitor the memory consumption.

jemalloc version

We can choose the jemalloc version by setting JEMALLOC_VERSION to a version number from this list. By default, the buildpack chooses the most recent version.


With just enabling jemalloc, we can see significant drop in memory usage on Heroku without doing any code change. So I highly recommend enabling it in production apps deployed on Heroku.